The present invention relates to a hydraulic knee joint.
Persons requiring an above-knee amputation lose both their knee and the remainder of the leg below the knee. An external prosthesis can serve to replace the lost function of the knee and foot. Modern prostheses are often modular in function, such that the knee and foot prostheses are separate units which work together to form an artificial limb. It is desirable for any artificial foot to be able to function in combination with any artificial knee joint. Thus an artificial knee joint should ideally provide for this modular function.
In use, an artificial knee joint must typically perform a stability function to provide stability when the user (patient) is standing and requiring the artificial limb to bear their weight or when the artificial limb is bearing weight during walking (stance phase), and must also be able to flex freely during the swing phase of the patient's gait.
Previously proposed artificial knee joints can be broadly classified as those providing stability via geometric stability, static friction stability or hydraulic stability. Control of the swing phase can be based on the use of springs, dynamic friction, pneumatic fluid resistance, hydraulic fluid, or hysteresis in solids.
The Regnell limb is an example of a hydraulic swing and stance controlled artificial knee, which is controlled by the position of the foot. It uses a cylinder piston arrangement, as described in SE 8,500,740-9, U.S. Pat. No. 4,662,486 and U.S. Pat. No. 4,775,037, in which the below-knee section of the artificial limb comprises an upper and a lower hydraulic chamber, each having a piston held on a common piston rod, arid the piston being pivotally connected to the artificial knee joint. If the flow of hydraulic fluid between the chambers is blocked, the knee joint is prevented from bending and the limb is locked in position. This can provide the required stability function so that the leg is locked in a substantially straight position during weight bearing. A control valve governing the flow between the hydraulic chambers to provide switching between hydraulic swing control and hydraulic stance control is proposed in GB-A-1380428. The control is provided by the position of the foot, an arrangement which requires a specific prosthetic foot to be used in conjunction with a specific prosthetic leg.
In line with modern requirements, attempts have been made to develop the Regnell limb into a modular system. U.S. Pat. No. 5,376,136 discloses a means of control of the hydraulic valve in the piston arrangement to provide modular function whereby the below-knee section comprises two hinged parts, the relative positions of which control the valve. Thus control is provided by the leg section alone, and not the foot, so that modularisation is achieved. However, the device has proved to be cumbersome, unreliable and subject to wear. In addition, when in use, the inertia of the foot and shoe during swing can result in motion of the below-knee section such as to cause locking of the knee joint into the stance mode necessary to support the patient when standing. This is clearly disadvantageous as the patient is thus prevented from taking further steps. Thus systems which rely on the position of parts of the artificial limb have generally proved unsatisfactory.
The so-called Catranis system is an alternative attempt to control the switching between stance and swing. It utilises an offset lever system to operate a valve arrangement. However, it too suffers from tendency to locking. The resistance of the hydraulic unit in motion during the swing phase produces a similar force pattern on the lever system to that produced by the application of the patient's weight when standing, with the result that the knee locks into the stance position during the swing phase.
Typically, the ratio of the forces generated in an artificial knee joint during swing phase and during stance phase is of the order of 1:5. The above mentioned systems fail to produce adequate control because during swing phase they are subject the large forces arising from the inertia of the foot and lower parts of the prosthesis.
An aim of the present invention is to address this disadvantage.
Accordingly a first aspect of the present invention is directed to a patient-weight-activated hydraulic knee joint comprising a hydraulic shaft system comprising a cylinder having an upper chamber and a lower chamber divided by a partition, a piston disposed within each chamber and dividing each chamber into a lower section and an upper section, a piston rod joining the pistons and passing through the partition, a first hydraulic line connecting the upper section of the upper hydraulic chamber and the lower section of the lower hydraulic chamber, and a second hydraulic line connecting the lower section of the upper chamber and the upper section of the lower chamber, and further comprising a frame holding the hydraulic shaft system, a knee attachment pivotally mounted on the frame, a crank shaft disposed within the knee attachment and having a pivot axis, a pivot pin disposed within the knee attachment with an axis parallel to that of the crank shaft to which the piston rod is pivotally connected and thereby transmits hydraulic force, force transmission means are connected to the crank shaft which transmit the force of a patient's weight when brought to bear on the knee attachment, in which a hydraulic valve is situated in the second hydraulic line and controlled by the force transmission system and hydraulic pressure such that combined application of the patient's weight via the force transmission system and the hydraulic pressure to the valve stops the flow of hydraulic fluid. This is advantageous in that control depends on application of the patient's weight so that locking occurs only when it is required, that is, during standing and those parts of the patient's gait when the limb is weight-bearing. Furthermore, the control is provided solely by features of the hydraulic knee joint, thus providing a fully modular unit.
Preferably, the cylinder is adapted at its lower end for attachment to a prosthetic ankle and foot.
Preferably, the knee attachment is adapted for attachment via suitable connectors to a leg stump.
Advantageously, auxiliary controls are provided to pre-set the level of patient weight required for the hydraulic valve to close during weight-bearing. Thus the hydraulic knee joint can be readily altered to suit the characteristics of a patient.
Preferably, the force transmission system comprises a lever system pivotally connected to the knee attachment at one end and at the other end to the valve.
Advantageously the lever system comprises a long lever pivotally fastened at one end to the crank shaft, a chain link pivotally fastened to the other end of the long lever, and a short lever pivotally fastened to the chain link. This provides an effective and straightforward way of transmitting the forces from the knee attachment to the valve.
Preferably, low friction means are provided to transfer the force of the patient's weight to the force transmission means. This allows efficient transfer of the force.
Advantageously the low friction means are roller bearings.
In a preferred embodiment the short lever controls the position of the hydraulic valve. This arrangement minimises the number of components of the force transmission system.
Advantageously, the movement of the short lever is constrained by auxiliary controls. Thus the movement required to operate the valve effectively can be adjusted if necessary.
Preferably, the pivot axis of the crank shaft is eccentric.
In a preferred embodiment the pivot pin axis is situated forwardly and downwardly of the crank shaft axis.
Advantageously, the eccentric pivot axis of the crank shaft and the pivot point of the knee attachment about the frame lie within a plane also containing a point in front of a patient's foot when the patient is using the patient-weight-activated hydraulic knee joint and standing upright. This arrangement reduces the tendency of hydraulic knee joint to lock into stance phase when the weight of the patient passes through the toe of the foot as compared to the heel of the foot.
In a preferred embodiment, the hydraulic valve comprises a hydraulic chamber containing a rotary shaft controlled by the force transmission system, and a piston moved by the rotary shaft.
Advantageously the hydraulic valve further comprises a valve seat which may be substantially sealed by the piston to close the second hydraulic line, the area of the piston being much greater than the area of the valve seat. This greater area provides a simple way of maintaining sufficient force during motion to keep the valve open and hence prevent the knee joint from locking.
Advantageously the hydraulic valve further comprises a passage leading from the chamber to the hydraulic line and containing a needle valve. This regulates the hydraulic flow.
Advantageously, the rotary shaft co-operates with a slot in the aide of the piston to effect movement of the piston. This provides a simple, low friction way of moving the piston.
Preferably the knee attachment comprises a substantially ball-like member.
A second aspect of the present invention is directed to a hydraulic trip valve for use in a hydraulic prosthetic joint comprising a flow passage between two hydraulic chambers and containing therein a first valve, a flow restrictor and a second valve which opens at a predetermined hydraulic pressure. The trip valve offers the advantage that flow through the flow passage can be prevented when the hydraulic pressure is below a predetermined level.
Advantageously the hydraulic trip valve is further provided with a damping member. This prevents the second valve from closing too quickly and thus reduces resonance effects.
Advantageously the second valve comprises a hydraulically damped piston.
Preferably the hydraulically damped piston is provided with a pressure relief ball valve which cooperates with the valve seat.
Advantageously the hydraulically damped piston is mounted on a spring having a spring force (S) which urges the valve to a closed position against a valve seat of area (A3).
In a preferred embodiment the spring force (S) and the valve seat area (A3) are predetermined according to P=S/A3 where P is the hydraulic pressure acting to open the valve arising from the gravitational forces acting when the joint is substantially at rest, Thus the parameters S and A3 can be selected to tailor the operation of the trip valve to the individual characteristics of the patient,
Preferably the flow restrictor comprises a needle valve. In this way the flow through the flow passage can be at a different rate in either direction.
Advantageously the damping member comprises a constriction in the flow passage.
Preferably the damping member is situated in a flow passage acting in parallel with the flow restrictor.
Preferably, the hydraulic trip valve further comprises a bypass valve which bypasses the valve seat. This allows the operation of the trip valve to be adjusted to balance the operation of the needle valve.
Advantageously, the patient-weight-activated hydraulic knee joint disclosed above further comprises such a hydraulic trip valve.